Potential of CMEMS products for assessing eutrophication status and natural variability in the north-eastern Baltic Sea.

Monitoring and assessment in the Baltic Sea region have been coordinated by HELCOM (Helsinki Commission) since 1979, with the first periodic assessment published in 1986. HELCOM latest assessments are based on indicators that reflect the achievement or non-achievement of good environmental status (GES) in the sub-basins of the Baltic Sea. For this purpose, HELCOM has developed a set of core indicators, with each of them having a quantitative GES threshold.

This comprehensive indicator-based system requires long-term and systematic measurements to minimize the influence of natural (spatial and temporal) variability on the trend estimates. State indicators and background hydrographic characteristics (as drivers of natural variability) have to be calculated and evaluated to distinguish pressure-induced changes/trends. However, routine environmental monitoring is carried out with a low spatial and temporal resolution. While model data could be used as a substitute, they are not applied yet largely due to shortcomings in accuracy and overall mistrust of model products.

We assessed the possibility of using the model and remote sensing data provided by Copernicus Marine Environment Monitoring Service (CMEMS) to calculate HELCOM eutrophication indicators and influencing hydrographic characteristics or extreme events like marine heatwaves and coastal upwelling events. Reanalysis products BALTICSEA_REANALYSIS_BIO_003_012 and BALTICSEA_REANALYSIS_PHY_003_011 together with remote sensing product SST_BAL_SST_L4_REP_OBSERVATIONS_010_016 were used for state indicators and hydrographic characteristics, respectively.

Selected HELCOM eutrophication indicators were calculated following HELCOM methodology as average concentrations in the surface layer (0 – 10 m) for winter (DIN and DIP), summer (Chl-a) and whole year (TN and TP). Indicator calculations show a steady decline in concentrations of all nutrient compounds in recent years for all basins in the north-eastern Baltic Sea that is not confirmed by the measurements. We suggest a way forward for harvesting monitoring data prior to their official submission deadline and producing interim reanalysis products to improve the confidence of assessments based on CMEMS products.

For occurrence and intensity of heatwaves, a climatology (1986-2020) of sea surface temperature (SST) and the 90^th percentile was calculated for each grid cell in the Baltic Sea. The heatwave was identified when SST exceeded the 90^th percentile value for a site and date. For upwellings, the SST data were analyzed along transects from coast to coast in either North-South or East-West direction. Every grid point with the local SST value >2 °C colder than the transect average was assigned to a coastal upwelling event. The results based on two selected products (reanalysis and remote sensing) agree well except in years/seasons when the seasonal thermocline was very shallow (e.g. 2018). We demonstrate that CMEMS products covering the surface layer dynamics in the Baltic Sea (e.g. SST) can be used in describing long-term trends and inter-annual variability in hydrographic conditions (also extreme events) and serve as background information for indicator-based eutrophication assessments.